Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information. The modulated photocurrent is a HF signal having a lowest fundamental frequency. The peak-to-peak value of the photocurrent is designated as I.sub.11, and the peak value of the HF signal associated with said frequency is designated as I.sub.peak, The modulation m is defined as: EQU m=I.sub.11 /I.sub.peak
and is proportional to the optical contrast M, which is defined as: EQU M=(R.sub.H -R.sub.I)/R.sub.H
wherein R.sub.H and R.sub.L are the reflections of the crystalline and amorphous material, respectively.
The main problems in phase-change optical recording are the required large number of overwrite cycles (cyclability), i.e. the number of repeated writing (amorphization) and erasing (recrystallization) operations, and a proper crystallization speed. High crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD-RAM and optical tape, where the complete crystallization time has to be shorter than 100 ns, preferably as short as 30 to 70 ns. If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) from the previous recording can not be completely removed (recrystallized) during DOW. This will cause a high noise level.
A well-known material for phase-change recording is based on Ge--Sb--Te. However, none of the known recording media satisfies all the requirements for the phase-change optical recording, in particular as regards the demands for cyclability and crystallization speed.
An optical information medium of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,289,453. The known medium of the phase-change type comprises a disc-shaped substrate carrying a stack of layers consisting, in succession, of a first dielectric layer, a recording layer of a phase-change Ge--Sb--Te alloy, a second dielectric layer and a metal reflective layer. Such a stack of layers can be referred to as an IPIM structure, wherein M represents a reflective or mirror layer, I represents a dielectric layer and P represents a phase-change recording layer. Said patent discloses a compound Ge.sub.x Sb.sub.y Te.sub.z for high speed recording (i.e. a relative linear speed of 8-13 m/s) which is located in an area JKLM in the triangular ternary Ge--Sb--Te composition diagram, which area has angular points J (Ge.sub.22.5 Sb.sub.22 Te.sub.55.5); K (Gel2.5Sb.sub.32 Te.sub.55.5); L (Ge.sub.14.5 Sb.sub.37.5 Te.sub.48); M (Ge.sub.26 Sb.sub.26 Te.sub.48). This area diverges from the tie-line connecting GeTe and Sb.sub.2 Te.sub.3. According to said patent, for high speed recording the thickness of the phase-change recording layer should range from 12 to 35 nm, preferably from 12 to 25 nm. Above a thickness of 25 nm, jitter, which is a measure of the distortion of the shape of a recording mark, increases. According to said patent, jitter does not increase to twice the initial value after repeating recording 10.sup.5 times.
The above citations are hereby incorporated in whole by reference.